This IS the Avid L2: [Avid-L2] Re: OT: Sony BVU 800 series TBCs color direct vs. color process?

Thursday, October 23, 2014

[Avid-L2] Re: OT: Sony BVU 800 series TBCs color direct vs. color process?

I did get the answer here but I just received more info from a fellow engineer I started with back in 1983. Just a fun overall review of the enginnering"

Hi John,

Wow I can't believe that was 31 (1983) years ago when we were dealing with these toys. It was fun, I still miss the mechanical aspect of VTRs though I do like being in the now with digital, file based HDTV.

Here is what I remember about the signal systems:

Color direct- If you had a TBC with SC 3.58 output and a deck that would take the SC, the TBC would send the "jittered subcarrier" to time base correct the chroma so that is was stable enough to be processed like a 1" type C deck. This was what was called color direct.

Advanced vertical in all variants of TBCs kept the video timed to the center of write memory. Cost effective TBCs at the time only had 16 lines worth of RAM memory. A frame store was a luxury at the time costing over $10K.

Process- The signal was decoded from the composite video into Y/R-y/B-y signals. Velocity errors & chroma time base errors were removed as part of the decoding process. Frequency H errors were then handled by the TBC write clock all together and A/D into the TBC buffers (memory). This was not a clean looking process since the decoders were terrible comb filters, 2H types with lots of picture H/V artifacts. This was used when you didn't have a TBC to handle 3/4" signal or were using a Quantel 1550 or NEC frame store to put 3/4" video on the air for net news applications. This was what 99% of the public saw of 3/4" ENG footage in their living rooms at the time.

Dub mode- This is the cleanest picture possible. It sort of worked like S-Video where the chroma was separate from the Y signal. The TBC-810 dub cable also sent the DOC pulse and looked at the RF to see any drop outs and correct them. It also dealt with the chroma at the 688KHz down converted stage, so the VTR demod did not have to re-encode a second time. If you had this setup with a BVU-800 then you had the cleanest video possible for 3/4" format. However this never realized its true potential because it needed an expensive color decoder on the record side to cleanly decode the composite video into Y-C 688 signal, and record to tape. If you remember Faroudja Labs had processors, but they were ridiculously expensive, $6K to $15K per machine. No one in their right mind would pay that for 3/4" format equipment. Better to stay with existing 1" type C, or BetaSP which came out in 1986.

---In avid-l2@yahoogroups.com, <bigfish@...> wrote :

Awesome, I'm having Acorn shaped waveform flashbacks. And by the way, you had me a COMB FILTER. ;-)

---In avid-l2@yahoogroups.com, <DennyD1@...> wrote :

On Oct 10, 2014, at 1:18 PM, bigfish@... wrote:

> It's the how it worked I'm trying to remember. The subcarrier feed back to the deck somehow neutralized the time base error of the chroma decode or some such thing. If you can fill in the blanks in my memory maybe I can let this go.

OK:

I'll try. The color-under system works by separating the color information from the luminance information. The process starts by filtering the 3.58MHz color info from the incoming composite video signal during recording. A simple high-pass filter would chop off and lose some of the higher-frequency luminance information, and would also include some luminance in the chroma, thereby corrupting the chroma information. So a better way to separate the two is by using a COMB FILTER, a special type of filter that uses the horizontal line frequency to 'comb out' the chroma from the luma. It's not a perfect separation, but it's better than a high-pass filter.
The luma is recorded on the tape as a frequency-modulated carrier starting at 3.8MHz, going up to about 5.4MHz. The filtered chroma information is down-converted from 3.58MHz to 688KHz and mixed with the luma's 3.8MHz FM carrier. This combined signal is what's recorded onto the tape. It's called 'color-under' because the color information is placed under the luminance information in frequency (Normal composite video contains the color info as quadrature amplitude-modulated sidebands of the 3.58MHz color subcarrier, which means that the color info is above the luminance info).
When you filter, down-convert, and separately modulate the luma and chroma (using two different modulation types), the two components can no longer be in any kind of timing relationship during playback. Normally with composite video, luma and chroma have a close phase relationship since the color subcarrier is in phase with the horizontal timing pulses of the video. Separating the two components destroys that relationship. This is necessary in order to allow color recording on 3/4-inch tape at 3 3/4-inches per second. But without a timing relationship between the two, they must be treated separately during playback if we hope to get quality results in a broadcast environment using a digital Time-Base Corrector or TBC. We want to avoid repeatedly separating and recombining the luma and chroma, since each time it's done, that process damages the integrity of the information. Usually during playback, a typical U-Matic VCR would simply up-convert the 688KHz chroma to an internally-generated 3.58MHz carrier and mix it together with the demodulated luma, producing a (non-coherent) composite video signal. This signal contains all the information, but the luma and chroma no longer have a locked timing relationship. The luma has its timing errors due to tape's mechanical instabilities and the chroma has its own instabilities due to its derivation from an up-converted 3.58MHz reference with no timing relationship to the luma.
If you simply take this non-coherent composite video and feed it into a digital TBC designed for this type of signal, the TBC will once again have to separate the luma and chroma (hopefully using a comb filter) in order to treat the two with separate timing error correction. Upon correction, the two will be combined together AGAIN to make composite video, but this output will be coherent. However, due to the additional processing, there will be still more luma detail loss and some of that detail will be in the chroma, causing ringing and false color edges, and possibly some chroma misalignment with the luma too. But at least the video itself will be a stable coherent signal.
Instead, if you use a BVU U-Matic VCR equipped with the special 'direct' cable connector and couple it with a suitable TBC also equipped with the same connector, the TBC can send a timing correction error derived from the luminance BACK to the VCR as a 'jittery' 3.58MHz reference, thereby counteracting the difference between the luma's and chroma's timing error. This effectively makes the two separate signals coherent again, which allows the TBC to process the two AS ONE composite signal (as it originally was) and avoids additional comb filtering.
How's that?

Dennis Degan, Video Editor-Consultant-Knowledge Bank
NBC Today Show, New York